"I can't upload an Instagram story: On my iPhone 15 pro max, I can easily add and upload a post shared by a person on Instagram to my story, but I cannot add any photo from the photos application saved on my device as a story."
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Ending App Errors:
A. Close All Apps:
An app is likely not responding. So, close all apps. Hold you finger down as you swipe up. Then, close all of these open apps, by swiping up quickly for all apps. Then, try this once more.
B. Perform a Force Restart:
Refer to this link: Force Restart iPhone - Apple Support